Bert says that a charged particle in a vacuum can travel in a helix only if a uniform electric field and a uniform magnetic field are both present and both parallel to the axis of the helix. Stuart says that only a magnetic field with a component parallel to the axis of the helix is needed. Which one, if either, is correct, and why?
a. Bert, because the charged particle's velocity can have a vertical component only if an electric field in the vertical direction is present.
b. Stuart, because a component of velocity in the vertical direction is not changed by a vertical component of a magnetic field. c. Bert, because a component of velocity in the vertical direction is changed by a vertical component of a magnetic field. d. Stuart, because an electric field in the vertical direction would cause the particle to come to a complete stop.
e. Neither, because particles cannot move in helical paths in the presence of magnetic and electric fields.
B
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The rotation of the Sun's photosphere is
a. fastest at the equator, slower at mid-latitudes, and slowest near the poles. b. slowest at the equator, faster at mid-latitudes, and fastest near the poles. c. fastest at the equator, slowest at mid-latitudes and the poles which travel at the same speed. d. the same regardless of latitude.
A neutron in a nuclear reactor makes an elastic head-on collision with a carbon atom initially at rest. (The mass of the carbon atom is 12 times that of the neutron.) What fraction of the neutron's kinetic energy is transferred to the carbon atom?
a. 14.4% b. 28.4% c. 41.4% d. 56.6%
The rate of energy flow per unit area of an electromagnetic wave has an average value of 0.695 W/m2. The wave is incident at right angles upon a rectangular area measuring 1.5 m by 2.0 m
How much total energy falls upon this rectangle each minute? (?0 = 8.85 × 10-12 C2/N ? m2, ?0 = 4? × 10-7 T ? m/A, c = 3.0 × 108 m/s) A) 130 J B) 160 J C) 190 J D) 220 J E) 250 J
In a curing process for plastic and other polymer-based materials, a plastic sheet, which initially is 25°C, is placed on a hot metal surface that is maintained at 250°C. If the back (or opposite) side of the 5-mm-thick plastic film is considered to be insulated, how long does it take for the back side to reach temperature of 90°C. Assume the following plastic properties: c = 1450 J/(kg K), k= 0.23 W/(m K), and ?= 1975 kg/m3
GIVEN
• Initial temperature of plastic sheet (T0)= 250C
• Sheet thickness (2L) = 5 mm = 0.005 m
• Opposite side or back is insulated
• Metal surface temperature (T?)= 2500C
• The thermal conductivity of the plastic (k) = 0.23 W/(m K)
• Density(?) = 1975 kg/m3
• Specific heat capacity (c)= 1450 J/(kg K)
FIND
Time taken for backside to reach temperature of 900C
ASSUMPTIONS
• The initial temperature of the sheet is uniform
• The temperature of the steel plates is constant
• The thermal conductivity of the sheet is constant
SKETCH
